In vehicles, there is a so-called hybrid vehicle having an internal combustion engine and a motor generator as a power source. In this hybrid vehicle, the engine is directly connected to the motor generator that drives and assists the engine. The motor generator is controlled to drive and assist by a controller based on states of driving of the hybrid vehicle and the engine. This basic arrangement is disclosed in JP Application Laid-Open No. 2000-170903.
In a conventional power regenerating controller for a hybrid vehicle, the hybrid vehicle is equipped with a motor (also referred to as “motor generator”) as well as an internal combustion engine to improve fuel economy. One example is disclosed in Japanese Application Laid-Open No. 2000-134713. In this application, a lockup clutch of a torque converter is controlled to be disengaged when regenerative braking is started or canceled. Thereby, the shock can be prevented, since the transferring of driving force is switched from mechanical to fluid transmission when the regenerative braking is started or stopped.
However, as shown in Japanese Application 08-105510, in a driven state of the engine, centrifugal force that is applied to hydraulic oil on an input side of the torque converter is smaller than that applied to hydraulic oil on an output side. Therefore the pressure to disengage the lockup clutch becomes higher than the pressure to engage. Accordingly, it is difficult to bring the lockup clutch to change from a disengagement state to an engagement state. To deal with this problem, beginning of fuel cut of the engine is delayed, or intake air quantity is increased temporarily by operation of a throttle valve so as to prevent the engine from the driven state to engage the lockup clutch.
If the regenerative braking is performed with the lockup clutch disengaged as in the above-mentioned Japanese Application Laid-Open No. 2000-134713, the engine speed sharply decreases by the added regenerative torque, and it is difficult to engage the lockup clutch again. Therefore, fuel cut must be canceled at an earlier stage, which undesirably increases fuel consumption.
Also, since the lockup clutch is not engaged, the engine speed sharply decreases and the regenerative electric power also undesirably decreases.
To deal with this problem, after beginning of the regenerative braking, the quantity of the intake air for the engine is controlled to maintain such that the lockup clutch of the torque converter can be engaged. That is, the intake air quantity should be increased by the quantity that the regenerative torque is added. However, this results in an undesirable increase in fuel consumption.